Magnetic particle core

ABSTRACT

Magnetic particle cores suitable for phase shifting applications having a permeability substantially above 275 and core losses below 0.24 ohms per henry per unit of inductance at 1800 Hertz are produced by a novel combination of steps including one or more of the following: reduction of pre-powder anneal additives, selective distribution of magnetic particle sizes, adding flexibility to the electrical insulation applied to the magnetic particles, elevated pressure compacting, surface etching of the pressure compacted core after hydrogen anneal, and an oxidation heat treatment following surface etching. Surface etching and oxidation combine to increase the breakstrength and improve other properties of various permeability cores, for example, from 125 through 300 permeability.

United States Patent [1 1 Laing, Alfred M.

[ Dec.4,1973

[5 MAGNETIC PARTICLE CORE [75] Inventor: Alfred MTLain g, Butler, Pa.[73] Assignee: Magnetics, lnc., Butler, Pa.

[22] Filed: Apr. 3, 1972 21 Appl. No.2 240,858

Related US. Application Data [62} Division of Ser. No. 80,476, Oct. 16,1970, Pat. No. 3,666,571, which is a division of Ser. No. 7l4,805, March21, 1968, Pat. No. 3,607,462.

[52] US. Cl 335/297, 336/233, l48/l04 [51] lm. CI. n01: 3/00 [58] Fieldof Search", 225/297; 336/233; 148/104 [56] References cited UNITEDSTATES PATENTS 6/1966 Opitz 148/104 X 3/1970 Copp l48/l04X PrimaryExaminerGeorge Harris Attorney-Raymond N. Baker [57] ABSTRACT Magneticparticle cores suitable for phase shifting applications having apermeability substantially above pacted core after hydrogen anneal, andan oxidation heat treatment following surface etching. Surface etchingand oxidation combine to increase the breakstrength and improve otherproperties of various permeability cores, for example, from 125 through300 permeability.

10 Claims, N0 Drawings MAGNETIC PARTICLE CORE This application is adivisional application of application Ser. No.'80,476, filed'Oct. 16,1970, now U. S.' Pat. No. 3,666,571, issued May 30, 1972, which was -adivisional application of application Ser. No. 714,805, filed Mar. 21,1968, now'U.S. Pat. No. 3,607,462, issued Sept. 21, 197 1.

This invention relates generally to preparation of magneticparticles'for compacting-into'magnetic components and, moreparticularly, to high permeability particle cores exhibiting-low corelosses inthe audio frequency and related frequency ranges, and-tomethods of manufacturing such cores.

A basic process for manufacture of particle cores is disclosed inthe-U.S.'Pat. to Bandur No.2,105,070. The process steps include thepreparation of magnetic particles, electrically insulating theparticles, compacting into a magnetic core, and annealing the core. Thisbasic process has been capable of producing molybdenumcontainingpermalloy particle cores having a penneability of about 125 and havingcore losses acceptable for audio-frequency applications.

Attempts to improve on this process have been numerous and continuoussince its origin. The problem is to increase the permeability withoutdestroying usefulness of the coredu'e to an increase in core losses.Successful improvements of core properties have generally resulted'fromtreatment of thecompactedcore, for-ex ample by boiling, solutiontreatment, and re-anneal of the compacted core as disclosed in the U.S.patents to Harendza-Harinxma No. 2,977,263 issued Mar. 28, 1961, No.3,014,825 issued Dec. 26, 1961, and 'No. 3,132,952 issued May 12, 1964.

The primary objective of particle core improvement endeavor is toincrease the permeability of such cores while maintainingcore losseswithin acceptable limits. Particle cores are used inelectricalcircuits-operating at voice frequencies and related frequencies up toabout 20,000 cycles per second where low core losses are a majorconsideration. Also these cores find certain applications at much higherfrequencieswhichfurther accentuate the importance of low core losses. Inthe audio frequency and related freq'uen'cyranges, componentsapproaching theoretical'perfect' reactance are desirable in orderto'obtain high quality performance. In filters, for example, shortercut-off, better defined resonance, and higher attentuationratios arerealized'with high quality inductors. Quality ininductors can beassessed from the Q factor. The Q of'induc'tors isdefined as a ratio vof reactance to resistance: Q nf w+ 4c)* f =frequency incycles'persecond L inductance inhenries, and

R wire resistance in ohms R Ac resistance (ohms-) due' to losses of thecore, in-

cluding eddy curre'nt losses, hysteresis losses, and.

wellknown in the-art, effortsto dateto improve on metallic particlecores have resulted in commercially available cores with-acceptablecorelosses having an upper limitof 200, and-slightly'higher,permeability. Special laboratory techniques may yield acceptable coresof'about 240 permeability; however, it has not beenv possible to producea particle core having a permeability above 275 with core lossesacceptable in the industry for the uses considered above.

The present invention teaches novelprocedures for preparation ofmagnetic powders for magnetic components and, specifically, preparationof molybdenumcohtaining permalloy powders forproducing magnetic cores ofhigher permeability, lower core losses, increased mechanicalbreakstrength, more linear temperature characteristic, and bettermoisture resistance. These teachings enable commercial production ofmolybdenum-containing permalloy powder cores having a permeabilitysubstantially above 275 and having c'ore losses within industryspecifications.

Cor e losses are reflected in the windings of a core as resistancelosses andconstitute loss of energy in an inductor. Core losses, as usedin the core manufacturing art, include eddy current losses'which varydirectly with the square of the frequency, hysteresis losses which varydirectly with the flux density and the frequency, and residual losseswhich'vary directly with the Residual loss Hysteresis loss Eddy currentloss Total loss factor 'R effective (total measured) resistance minus DCresistance p. permeability L inductance (henries) f frequency (Hz) 8,,flux density (gausses) e eddy current loss coefficient a hysteresis losscoefficient c residual loss coefficient To meet industry standards,cores must have a total coreloss no higher than 0.240 units (ohms perunit permeability per unit inductance) at 1800 Hertz (cycles persecond), with a core loss of 0.200 units at 1800 Hertz beingthe acceptedaverage.

To meet these standards and improve magnetic and mechanical propertiesas well, several novel steps are combinedin the present invention. Thesesteps will be 'described'in relation to production of novel highpermeability molybdenum-containing permalloy powder cores withacceptable core losses; Previously, the highest permeability core, ofthe type discussed, available commercially was a 200 perm" core. Thepresent teachings made the 300 perm core commercially aVailableJOfspecial significance is the fact'that these 300"perm cores have lossesacceptable within industry standards for audio-frequency uses, i.e.below 0.240 units.

78.0-83.0% the balance iron.

molybdenum, nickel, and

Methods for pulverizing the alloy are known. In practice the metallicconstituents of the alloy are melted together and additives whichembrittle the alloy are made in the molten state. This treatmentfacilitates a fine crystalline structure in the solidified alloy thatenables reduction to a fine powder byconventional rolling, grinding, andpulverizing techniques.

In departing from prior practice, the invention teaches selection of avariety of particle sizes to obtain an optimum packing factor, i.e. anoptimum density of magnetic powder and optimum space for electricalinsulation (electrically equivalent to air space) in the compactedproduct.

A typical particle size distribution to produce 300 perm cores inaccordance with the process of the present invention is:

TABLE I Average Particle Size Sieve by Microns Mesh-size Weight 90 froml20 to +230 ll5% 65 from -230 to +400 2535% 37 (or less) -400 45-65% lto +230 covers particle sizes which will pass through a 120 mesh screenbut will not pass through a 230 mesh screen using normal sieve practice.

The important concept discovered here is that the optimum packing factorto obtain high permeability and acceptable core losses is not obtainedby use of single particle size screening but rather by selectivescreening and distribution of particle sizes. The high permeabilityacceptable core loss product of the invention can be obtained byselecting particles with about one part by weight having an averageparticle size of 90 microns, about three parts by weight having anaverage particle size of 65 microns, and about six parts by weight inwhich the average particle size is no greater than about 37 microns.These proportions can be broadened to emphasize certain core propertiesand changes in the electrical insulation also permit variation in theseproportions. However, a larger overall-average particle size tends toincrease both permeability and core losses while smaller overall-averageparticle size tends to'decrease both permeability and core losses. I

After pulverizing and sieving, the powder is annealed to relieve thestrains induced during brittle practice,

that is during the production of the powder. To. prevent welding of theparticles during this anneal, additions of non-agglomerating materialmust be blended with the powder. Such material must remain non-reactiveor inert at powder annealing temperatures. In.theprior art, pre-annealadditions constituted about 0.3 to 1.0 percent by weight of the metalpowder. An important discovery of the present invention relates tobetter use of the limited amount of distributed non-magnetic gapavailable in producing the higher permeability product of the presentinvention; that is, this space can be better used to provide moreeffective electrical insulation of the particles rather than beingoccupied by pre-anneal additives when proper steps are followed. By theprocedures of the present invention, the pre-anneal additive isdrastically reduced to about 0.02 to about, 0.'05 percent by weight ofthe metallic powder; preferably such additives are held below about 0.03percent by weight. Typically ceramic clays such as talc or kaolin areadded to prevent agglomeration; a preferred preanneal additive ispowdered kaolin.

The subsequent powder anneal is held to a temperature of about 1250F.for about 1 7% hours in a nonoxidizing atmosphere, e.g. an atmospherecontaining free hydrogen. Temperatures significantly higher than about l250F. are avoided in order to eliminate agglomeration of the metalparticles. With the present invention it is possible to avoid the 1400F.to l600F. powder anneals of the prior art without sacrificing electricalproperties. In fact, a higher permeability low core loss product isobtained.

During the powder anneal the water of crystallization of the kaolin,which constitutes about 13 percent by weight of this pre-annealadditive, is driven off. The kaolin should be in the uncalcinedcondition before the powder anneal since it has been found that calcinedkaolin is not as effective as standard kaolin in preventingagglomeration of the metal particles.

After annealing, the work product is sieved through a 50 to mesh screento remove any lumps which may be formed; however, this does not changethe analysis of the metal powder sizes which have been selected toproduce the desired packing factor. This sieving merely breaks up looselumping which may occur.

The magnetic powder is then electrically insulated utilizing a slurryincluding, by present practice, a silicate, an inert metallic oxide, anda ceramic clay. For example, a solution containing about 67 grams ofsodium silicate, about 100 grams of milk of magnesia, and about 6000 cc.of deionized water is prepared to insulate about 50 pounds of powder.The electrical insulation is applied in a plurality of coats with thefirst coat ordinarily not. including a ceramic clay additive in order toutilize the pre-anneal non-agglomerating additive present with the metalpowder. Subsequent coats after the first coating, utilize about 1400 ccsof the above solution with about 22 grams of powdered kaolin added. Inpreferred practice the electrical insulation is applied in four separatecoats with intermediate dryings of the coatings being carried out attemperatures uy to about 315F. The total electrical insulation, dryweight, is less than about 0.4 percent by weight of the metal powderweight.

Highly beneficial results are obtained by the addition of a plasticizercoating during the insulation process. Plasticizer as used herein,refers to a material for imparting flexibility to the electricalinsulation or a major ingredient of the electrical insulation, e.g. themetallic silicate in the disclosed insulation. As taught by the presentinvention, the plasticizer must maintain this capability of impartingflexibility to the electrical insulation. during processing steps up toand including the compacting step.

Preferably the plasticizer is added as a final coating to the insulatedparticles; however plasticizers exist which require intermediate coatingfor best results. Suitable plasticizers of the latter category includestarches, sugars; or glycerin and a wetting-agent. A plasticizersuitable for addingas a final coatingis ammonium lignosulfonate-inaliquidcarrier.

The purpose of the plasticizer is to impart flexibility to theelectrical insulation and-permit higher than usual pressures duringcompacting. of the particles while avoidingmechanical cracking of theelectrical insulation. In accordance with'the teachings of theinvention,the plasticizer should maintain its capabilit'yof imparting flexibilityduring the temperaturesencountered in applying electricalinsulation-andthose encounteredin compacting. Preferably the-plasticizershouldbe driven off during the high temperature coreanneal or, atleast,not impair the insulation or leave a reaction product having reducedelectrical insulation properties.-

After the insulation process,.including.the useof 'a Y plasticizer, theinsulated powder is sieved through a 50 to 100 meshscreen to removelumps and chips of'insulation. This sieving-is carried out withoutchanging the. basic magnetic particle sieveanalysis.

The insulated powder is pressed into cores at a pressure which is.significantly higher. than that previously specified formolybdenum-containing permalloy powder cores-The compacting pressuretaught by the present invention for the production of higherpermeability cores is ordinarily in the rangeof about 135m 150 tons persquare inch'and preferably .is about 140 tons per square inch.Theplasticizer'makes the insulation 'more'. flexible and reducescompacting friction.

Without a plasticizer, the core losses-increase considerably at thehigher compacting pressures'taughtPA" portion of the decrease in corelosses available with the present invention can be traced to thedecrease in surface weldingstemmingfromthe decrease in'compactingfriction. The reduction in core losses also stems from decreasingmechanical breakage of the electrical insulation between particles whichapparently existed in the prior art practice. Further, the plasticizerreduces the amount'of lubricant needed in pressing.

However there are limits to the amount of plasticizer which can besafely used since mechanical breakstrength of the core decreases rapidlyabove certain low'level percentages. When ammonium lignosulfonate isused with an insulation containing sodium silicate, the amount of 'dryplasticizer should be about 0.06 percent by weight of the-metal powderweight.

The inventionincludes-discovery of a step to maintain desired mechanicalbreakstrength of the finished product. The co-action of this step, whichwill be described later, offsets any weakening effect on the corescaused by the plasticizer so that cores with mechanical breakstrengthequivalent to prior art cores without plasticizer can now bemadenotwithstandingthe use of a plasticizer.

After pressing, the cores are annealed between about l000F. and about1500F., preferably about 1250 F. for approximately-40 minutes inanon-oxidizingatmo-v sphere, for example, an-atmosphere containing purehydrogen. The cores'are quenched in water after removal from theannealing furnace.

Practice of the process of the invention described thus far consistentlyproduces cores within the normal tolerance range for 300permeabilitycores, however face etching step: By surface etching ismeant removal of the skin effect resulting from present-day compacting.techniques used in commercial production of molybdenum-permalloy cores.i

The cores are surface etched subsequent to the hydrogen anneal whichfollows pressing. The cores should not be surface etched prior to thisanneal. In general, chemical etching is preferred inorder to avoidadding any mechanical-strains to the particles. A typicaletchingpractice utilizes a 50 percent nitric acid solution with anetching time of 20 seconds, plus or minus 5 seconds with temperaturemaintained at F. 5F. An alternate etching procedure is approximately 3minutes in40 Baume nitric acid with temperature maintained at 80F i 5F.

Surface etching can cause a slight decrease in permeability but thisdecrease is limited to about 0.5 to about 5 percent of the corepermeability. Typically, a 302.1 permeability core may be reduced to300.6 permeability and a 323.3 permeability core may be reducedto 317.3.However, corelosses decrease at a much greater rate than permeability;decreases in core losses up to about 50 percent-are typical. Forexample, the above 302.1 permeability core had a AR/uL value of 0.117before surfaceetching, This core loss was reduced to 0.0972 by surfaceetching. The above 323.3 permeability core had a AR/uL value of .413units before etching which was reduced to 0.203 units, more than 50percent, by surface etching. In brief, while the permeability may bedecreased as-much as 5 percent by surface etching, the-core losses arereducedas much as 50 percent.

While 300 permeability cores with core losses within accepted standardscan be produced consistently in production utilizing the above steps,the invention also includes discovery of a novel step in the treatmentof compacted cores which further improves electrical and mechanicalproperties ,of molybdenum-containing permalloy'particle cores. This stepco-acts with other steps in the production of 275 and higherpermeability cores- For example, this step helps increase the mechanicalstrength of a high permeability core. offsetting any weakening effect ofa plasticizer coating. This step also acts to offset the effects of thehigher pressures used in producing 275 and higher permeability cores bydecreasing core losses which could result from such higher pressures.However this novel step also improves the mechanical and electricalproperties of lower permeability cores as well.

In accordance withthe invention, after annealing of the powder cores ina non-oxidizing atmosphere such as hydrogen, and after the surfaceetching, the cores are heat treated in an oxygen-containing atmosphere,for example, air. The order of these two heat treatments, that is thehydrogen anneal and the air heat treatment, cannot be reversed withoutloss of the benefits obtained by annealing in hydrogen, followed bysurface etching,.followedby oxidation. It is believed that a hydrogenanneal subsequent to the heat treatment in air reduces the bonds formedduring oxidation.

Theoxidation step should be carried out at a temperature between about600F. and about 1000F. for an interval of about 10 to 15 minutes. Apreferred oxidation treatment is applied at about 850F. for about 15minutes. It should be understood that this oxidation treatment has atime-temperature relationship, that is, a longer period of time, forexample, 1 Va hours at a lower temperature, for example about 225F., canbe utilized to provide similar oxidation, but the time involved isuneconomical and can have slightly detrimental side effects on otherproperties. In general, within the above limits, the improvement inbreak-strength is greater at higher temperatures.

Oxidation increases the breakstrength of the core as much as 75 percentdepending on the particular core, decreases total core losses as much asT 25 percent (chiefly a decrease in eddy current losses), and markedlydecreases the effects of moisture on a core.

Certain benefits of oxidation heat treatment are more pronounced withhigher permeability cores. A 300 permeability toroidal core, unoxidized,having a 1.06 inch outer about 0.5 inch inner diameter, and 0.44 inchheight, breaks at a 170 pounds of force per square centimeter of radialcross sectional area. An otherwise identical core, oxidized at about850F. for 12 minutes, breaks at 260 pounds per square centimeter, anincrease in break-strength of 50 percent. The breakstrength, however, ofa 1 15 to 135 permeability core of similar size shows an increase inbreakstrength of about 10 percent when treated in the same fashion.

Breakstrength measurements are made in accordance with the industryaccepted Vertical Core Breakstrength Test. This is a mechanical test inwhich force is applied on diametrically opposite sides of a paintedcores outer diameter with maximum tangential contact being made on bothsides. The ramming force required to break the core is measured inpoundsper square centimeter of a radial cross section of the core.

This test provides an important parameter for designating a mechanicalcharacteristic, that is the breakstrength, of the product of the presentinvention. 1f the breakstrength measured in accordance with the VerticalCore Breakstrength Test is plotted versus crosssectional areas of aradial segment of cores in square centimeters a linear relationship isfound to exist. The minimum acceptable value of this ratio of pounds ofbreakstrength to area of a radial segment in square centimeters, for apainted core, is 290. For example, a core with an outer diameter of 1.06inches, an inner diameter of 0.580 inch, and a height of 0.440 inch hasa radial section area of 0.635 square centimeters. The minimum acceptedbreakstrength of a painted core of this size is 184.15 pounds. Thebreakstrength factor of a core having this minimum breakstrength wouldbe 184.15 divided by 0.635 which equals 290. The electrically insulatingpaint applied to the exterior as a final step in processing particlecores may be conventional, e.g. an enamel core paint with a thickness ofroughly 7 to 12 mils.

All molybdenum-containing permalloy cores from 125 permeabilityv to 300permeability show a substantial decrease in core losses after theoxidation heat treatment taught. The higher permeability cores show bestresults when annealed in the range of roughly 750 to 850F. for about 12minutes. However with all cores of the type described, if thetemperature of the oxidation heat treatment is allowed to rise aboveabout 1000F., the cores will deteriorate with regard to losses.

Oxidation helps solve a problem of long standing in this art, that isthe detrimental effect of humidity o'n permeability; see StabilityCharacteristics of Molyb denum Permalloy Powder Cores" by C. D. Owens,Electrical Engineering, March 1956, pages 252-255. Past efforts havebeen concentrated on finding and applying coatings and packings forcores which would from the point of view of practical handling problemsand economics, have been at the limits of their capability for sometime. The oxidation step taught by the invention helps to solve thisproblem in the core itself and, for the first time in this art, bringsthe humidity problem under practical and economic control.

Humidity decreases the permeability of a core. The oxidation treatmenttaught by the invention reduces this decrease in permeability by atleast 50 percent in all cores and by greater amounts in the higher (300)permeability cores disclosed. For example, an unoxidized 300permeability core, with conventional electrical insulating enamelcoatings of about 7 to 12 mils totalthickness on its exterior surface,shows a change of 3.3 percent in permeability when exposed to percentrelative humidity in air at F. for five days. Otherwise identical cores,oxidized between 575F. and 850F. for twelve minutes had a permeabilitychange of 1.0 percent. Under the same conditions 200 permeability coresunoxidized showed a 2.2 percent change in inductance in this test whilethe oxidized cores showed an average change of l.1 percent inpermeability.

Also the effect of changes in temperature on permeability, i.e. thechange in permeability versus change in temperature characteristic, ismade more linear. This is partially due, it is believed, to a reductionin the effect of the difference in coefficients of expansion between thepaint on a core and the core itself, especially at lower temperatures.Evidently an oxidized core is better able to withstand the force ofcontraction of the paint on the core because of the increasedbreakstrength resulting from oxidation.

Improvement in breakstrength due to oxidation is especially beneficialwith the higher permeability cores compacted from electrically insulatedparticles having a plasticizer coating. A plasticizer dry coating weightof 0.06 to 0.1 percent ammonium ligno-sulfonate significantly improvescore losses but decreases the breakstrength of such cores slightly.Surface etching and heat treating in air cause equivalent or higherbreakstrength than that experienced with conventional cores of the samesize without a plasticizer.

The following table lists permeability and core losses, obtained incommercial production of 300 permeability core (normal tolerance :8percent permeability), for the various standard core sizes, using theteachings of the present invention.

TABLE I1 OD(in.) lD(in.) Ht(in.) p. AR/uL 0.250 0.110 0.110 281 0.1700.310 0.156 0.125 307 0.121 0.400 0.200 0.156 308 0.120 0.500 0.3000.187 310 0.190 0.900 0.550 0.300 277 .130 1.060 0.580 0.440 298 0.1281.300 0.785 0.420 292 0.130 1.410 0.880 0.412 298 0.146 1.570 0.9500.570 308 0.210

In describing specific embodiments of the invention, detailed steps,values, and determinations have been set forth which not only enablepractice of the invention but also provide guidelines for modificationof the specific embodiments by those skilled in the art, therefore thescope of the invention is to be determined from the following claims.

I claim:

1. A pressure compacted magnetic core comprising magnetic particles anddistributed non-magnetic gaps, the magnetic core having a permeabilityabove 275 units and a core loss when operated at a frequency of 1800Hertz no greater than 0.240 ohms per henry per unit of permeability withcore loss being measured in accordance with the following formula:

R /uL ef aB f cf wherein R is the total AC core losses in ohms, equal tothe effective resistance minus the DC resistance losses L is theinductance in henries u is permeability (above 275 units) T3,, is theflux density (20 gausses) fis the frequency (1800 Hertz) e is the eddycurrent resistance coefficient having a maximum value of 46.7 X v a isthe hysteresis resistance coefficient having a maximum value of 1.3 X10", and

c is the residual loss coefficient having a maximum value of 23 X 10 2.The magnetic core of claim 1 in which the magnetic particles comprise amolybdenum-containing permalloy consisting essentially of molybdenum,nickel, and iron.

3. The magnetic core of claim 2 in which the magnetic particles have thefollowing particle size distribution:

about 1 part by weight average particle size about 90 microns,

about 3 parts by weight average particle size about 65 microns, and

about 6 parts by weight particle size average not greater than about 37microns.

4. The magnetic core of claim 2 in which the magnetic metallic particlesare separated by electrical insulation comprising the reaction productof a metallic silicate, an inert metallic oxide, a ceramic clay, and aplasticizer with the plasticizer comprising less than 0.1 percent dryweight of the magnetic metallic particles.

5. The magnetic core of claim 4 in which the electrical insulationincludes sodium silicate, magnesium oxide and kaolin.

6. The magnetic core of claim 1 in which the permeability is at least300.

7. The magnetic core of claim 1 having a toroidal configuration with itsexterior coated with an electrically insulating paint and a mechanicalbreakstrength such that a ratio of the pounds of force required to breakthe core to the area in square centimeters of a radial cross-section ofthe core has a minimum value of 290, the breakstrength of the core beingmeasured by applying the force to diametrically opposite sides of theouter diameter of the core.

8. The magnetic core of claim 1 characterized by improved resistance tohumidity such that after conventional exterior electricalinsulationcoating it exhibits a decrease in permeability of less than 2percent when exposed to at least 95 percent relative humidity in air at150F. for five days.

9. A pressure compacted magnetic core comprising magnetic particles anddistributed non-magnetic gaps, the magnetic core having a permeabilityabove 115 units and a core loss when operated at a frequency of 1800Hertz no greater than 0.240 ohms per henry per unit of permeability withcore loss being measured in accordance with the following formula:

R L ef aB f cf wherein R is the total AC core losses in ohms, equal tothe effective resistance minus the DC resistance losses L is theinductance in henries y. is permeability (above 275 units) 13,, is theflux density (20 gausses) f is the frequency (1800 Hertz) e is the eddycurrent resistance coefiicient having a maximum value of 46.7 X 10' a isthe hysteresis resistance coefficient having a maximum value of 1.3 X 10and c is the residual loss coefficient having a maximum value of 23 X10', and further characterized by improved resistance to humidity suchthat after conventional electrical insulation coating of its exteriorsurface it exhibits a decrease in permeability of less than 2 percentwhen exposed to at least percent relative humidity in air at 150F. forfive days.

10. A pressure compacted magnetic core comprising magnetic particles anddistributed non-magnetic gaps, the magnetic core having a permeabilityabove units and a core loss when operated at a frequency of 1800 Hertzno greater than 0.240 ohms per henry per unit of permeability with coreloss being measured in accordance with the following formula:

R L ef aB f cf wherein R is the total AC core losses in ohms, equal tothe effective resistance minus the DC resistance losses L is theinductance in henries [L is permeability (above 275 units) B,, is theflux density (20 gausses) f is the frequency (1800 Hertz) e is the eddycurrent resistance coefficient having a maximum value of 46.7 X 10' a isthe hysteresis resistance coefficient having a maximum value of '1.3 X10', and c is the residual loss coefficient having a maximum value of 23X 10 the magnetic corehaving a toroidal configuration with its exteriorcoated with an electrically insulating paint, and further characterizedby improved mechanical breakstrength such that a ratio of the pounds offorce required to break the core to the area in square centimeters of aradial section of the core has a minimum value of 290, the breakstrengthof the core being measured by applying force to diametrically oppositesides of the outer diameter of the core.

2. The magnetic core of claim 1 in which the magnetic particles comprisea molybdenum-containing permalloy consisting essentially of molybdenum,nickel, and iron.
 3. The magnetic core of claim 2 in which the magneticparticles have the following particle size distribution: about 1 part byweight average particle size about 90 microns, about 3 parts by weightaverage particle size about 65 microns, and about 6 parts by weightparticle size average not greater than about 37 microns.
 4. The magneticcore of claim 2 in which the magnetic metallic particles are separatedby electrical insulation comprising the reaction product of a metallicsilicate, an inert metallic oxide, a ceramic clay, and a plasticizerwith the plasticizer comprising less than 0.1 percent dry weight of themagnetic metallic particles.
 5. The magnetic core of claim 4 in whichthe electrical insulation includes sodium silicate, magnesium oxide andkaolin.
 6. The magnetic core of claim 1 in which the permeability is atleast
 300. 7. The magnetic core of claim 1 having a toroidalconfiguration with its exterior coated with an electrically insulatingpainT and a mechanical breakstrength such that a ratio of the pounds offorce required to break the core to the area in square centimeters of aradial cross-section of the core has a minimum value of 290, thebreakstrength of the core being measured by applying the force todiametrically opposite sides of the outer diameter of the core.
 8. Themagnetic core of claim 1 characterized by improved resistance tohumidity such that after conventional exterior electrical insulationcoating it exhibits a decrease in permeability of less than 2 percentwhen exposed to at least 95 percent relative humidity in air at 150*F.for five days.
 9. A pressure compacted magnetic core comprising magneticparticles and distributed non-magnetic gaps, the magnetic core having apermeability above 115 units and a core loss when operated at afrequency of 1800 Hertz no greater than 0.240 ohms per henry per unit ofpermeability with core loss being measured in accordance with thefollowing formula: RAC/ Mu L ef2 + aBmf + cf wherein RAC is the total ACcore losses in ohms, equal to the effective resistance minus the DCresistance losses L is the inductance in henries Mu is permeability(above 275 units) Bm is the flux density (20 gausses) f is the frequency(1800 Hertz) e is the eddy current resistance coefficient having amaximum value of 46.7 X 10 9 a is the hysteresis resistance coefficienthaving a maximum value of 1.3 X 10 6, and c is the residual losscoefficient having a maximum value of 23 X 10 6, and furthercharacterized by improved resistance to humidity such that afterconventional electrical insulation coating of its exterior surface itexhibits a decrease in permeability of less than 2 percent when exposedto at least 95 percent relative humidity in air at 150*F. for five days.10. A pressure compacted magnetic core comprising magnetic particles anddistributed non-magnetic gaps, the magnetic core having a permeabilityabove 115 units and a core loss when operated at a frequency of 1800Hertz no greater than 0.240 ohms per henry per unit of permeability withcore loss being measured in accordance with the following formula: RAC/Mu L ef2 + aBmf + cf wherein RAC is the total AC core losses in ohms,equal to the effective resistance minus the DC resistance losses L isthe inductance in henries Mu is permeability (above 275 units) Bm is theflux density (20 gausses) f is the frequency (1800 Hertz) e is the eddycurrent resistance coefficient having a maximum value of 46.7 X 10 9 ais the hysteresis resistance coefficient having a maximum value of 1.3 X10 6, and c is the residual loss coefficient having a maximum value of23 X 10 6, the magnetic core having a toroidal configuration with itsexterior coated with an electrically insulating paint, and furthercharacterized by improved mechanical breakstrength such that a ratio ofthe pounds of force required to break the core to the area in squarecentimeters of a radial section of the core has a minimum value of 290,the breakstrength of the core being measured by applying force todiametrically opposite sides of the outer diameter of the core.